43 
its real argillaceous part. This earth readily combines with acids, 
and forms the salt called alum, by its union with the sulphuric acid 
or the acid of sulphur. It hardens to a very considerable degree 
in the fire ; but does not fuse, unless combined with lime, or certain 
salts containing the phosphoric acid. The other ingredient, in the 
formation of clay, is silicious sand, of which I shall presently speak 
more fully. From the various proportions of these two substances ; 
from the different degrees of their respective purity ; and, particu- 
larly, from the admixture of iron, and of some of the other earths, 
proceeds a considerable variety in clays. Those which are most 
likely to come under our observation are, the lithomarga, formed by 
the finer clays in various degrees of induration ; and fuller s earth, 
an argillaceous substance, deriving a saponaceous softness from its 
containing a small portion of magnesia. 
Trap, rowley rag, toad stone, or wacken, are stones in which ar- 
gillaceous matter very much predominates. These, from the manner 
in which they are diffused through the subterranean regions, and 
from the peculiar structure which they possess, have been erro- 
neously supposed to be of volcanic origin. 
Schistus, or slate, is also a mixture, in which alumine very much 
predominates. It also contains the earth of flint and magnesia. It 
is found to vary considerably in colour and texture, according to 
the degrees of purity, and the different proportions of its constituent 
parts. 
Silica, or silicious earth, the earth of flints, is the chief consti- 
tuent of the pebbles and sand, which form the third stratmn I have 
mentioned. It is difficultly acted on by any acid, except that which 
is obtained by distillation from Derbyshire fluor spar ; and which is 
termed fluoric acid. It is, however, very powerfully acted on by the 
alkalies, which promote its fusion, and, when thus combined with 
it, in a certain degree, form glass. It is this earth, which chiefly 
V 
